Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive. To improve the economics of natural gas use, much research has focused on the use of methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids, which are more easily transported and thus more economical. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is converted into a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted into hydrocarbons.
This second step, the preparation of hydrocarbons from synthesis gas, is well known in the art and is usually referred to as Fischer-Tropsch synthesis, the Fischer-Tropsch process, or Fischer-Tropsch reaction(s). Fischer-Tropsch synthesis generally entails contacting a stream of synthesis gas with a catalyst under temperature and pressure conditions that allow the synthesis gas to react and form hydrocarbons.
More specifically, the Fischer-Tropsch reaction is the catalytic hydrogenation of carbon monoxide to produce any of a variety of products ranging from methane to higher alkanes and aliphatic alcohols. Research continues on the development of more efficient Fischer-Tropsch catalyst systems and reaction systems that increase the selectivity for high-value hydrocarbons in the Fischer-Tropsch product stream.
Originally, the Fischer-Tropsch synthesis was carried out in packed bed reactors. These reactors have several drawbacks, such as temperature control, that can be overcome by gas-agitated slurry reactors or slurry bubble column reactors. Gas-agitated multiphase reactors sometimes called “slurry reactors” or “slurry bubble columns,” operate by suspending catalytic particles in liquid and feeding gas reactants into the bottom of the reactor through a gas distributor, which produces small gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are typically converted to gaseous and liquid products. The gaseous products formed enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered from the suspending liquid by using different techniques like filtration, settling, hydrocyclones, magnetic techniques, etc. Gas-agitated multiphase reactors or slurry bubble column reactors (SBCRs) inherently have very high heat transfer rates; therefore, reduced reactor cost and the ability to remove and add catalyst online are principal advantages of such reactors in Fischer-Tropsch synthesis, which is exothermic. Sie and Krishna (Appl. Catalysis A: General 1999, 186, p. 55) give a history of the development of various Fischer Tropsch reactors and the advantages of slurry bubble columns over fixed bed reactors.
It is clear from the prior art that the performance of a SBCR is a combined result of reaction kinetics, heat and mass transfer, and multiphase hydrodynamics. Jackson, Torczynski, Shollenberger, O'Hern, and Adkins (Proc. Annual Int. Pittsburgh Coal Conf. 1996, 13th (Vol 2), p. 1226) showed experimental evidence of the increase of gas hold up with increase in the inlet superficial velocity in a SBCR for Fischer Tropsch synthesis. Krishna, DeSwart, Ellenberger, Martina, and Maretto (AIChE J. 1997, 43(2), p. 311) measured experimentally the increase in gas holdup with an increase in the gas velocity and solids concentration in a slurry bubble column in chum turbulent regime. Letzel, Schouten, Krishna and van den Bleek (Chem. Eng. Sci 1999, 54, p. 2237) developed a simple model for gas holdup and mass transfer at high pressure in a slurry bubble column. Numerically, Sanyel, Vasquez, Roy, and Dudukovic (Chem. Eng. Sci. 1999, 54, p. 5071) and Pan, Dudukovic, and Chang (Chem. Eng. Sci. 1999, 54, p. 2481) showed examples of computational fluid dynamic modeling and optimization of a slurry bubble column reactor irrespective of the chemistry. Wu and Gidaspow, (Chem. Eng. Sci 2000, 55, p. 573) show examples of computational fluid dynamics simulations of hydrodynamics of Slurry Bubble Column processes.
Much previous work has been aimed at optimization of the slurry bubble column system for Fischer Tropsch and other chemistries. Stem et al. (Ind. Eng. Chem. Process Des. Dev. 1985, 25, p. 1214) developed an axial dispersion model for describing the performance of gas agitated multiphase reactor used for Fischer-Tropsch synthesis. Saxena (Cat. Rev. -Sci. Eng. 1995 37, p. 227) gives a review of the detailed experimental findings and theoretical models for the design of a Fischer Tropsch SBCR. It is clear from all the work in industry and academia that there is a need for an optimized Fischer Tropsch reactor and reactor configuration.
Considerable patent literature addresses the optimization of the Fischer Tropsch Slurry Bubble Column reactor (SBCR) and the overall system. U.S. Pat. No. 5,252,613 presents a method for improving catalyst particle distribution by introducing a secondary suspending fluid. U.S. Pat. No. 5,348,982 shows an optimal mode of operation for SBCR. U.S. Pat. No. 5,382,748 shows the use of a vertical downcomer to promote the uniform catalyst distribution. U.S. Pat. Nos. 5,961,933 and 6,060,524 show that optimal operation can be obtained by introduction of liquid recirculation.
The flow patterns of individual phases will affect the reactor performance. The plug flow and well-mixed flow are two extreme flow patterns for reactor systems. The dimensionless number, Pe, can be used to represent the degree of backmixing in plug flow. It is noted by Deckwer (Chem. Eng. Sci. 1976, 31, p. 39) that the gas dispersion is important in bubble columns of diameters greater than 0.5 m as it may have a strong influence on conversion. It is found that the gas dispersion is a function of the gas holdup, superficial gas velocity, and reactor diameter. In the gas-liquid-solid three phase reactor, the gas holdup depends on many factors such as gas and liquid velocities, gas distributor design, column geometry, physical properties of the gas and liquid, particle concentration, and reactor internals. Therefore, the gas dispersion coefficient is also a complicated function of these design and operating parameters. Usually, it is necessary to perform an in situ measurement to determine the dispersion coefficient at a given condition.
U.S. Pat. No. 5,348,982 described the plug flow to be the preferred optimum operating condition for the Slurry Bubble Column reactor in Fischer-Tropsch synthesis. The '982 patent teaches that the gas velocity should be larger than 0.2 DG/H, which corresponds to the gas Peclet number larger than 0.2. The concept taught in the '982 patent is that the reactor volume required to achieve a high conversion using plug flow is significantly less than the volume required using well mixed flow. It is clear that the plug flow favors the high conversion for the FT synthesis. However, the extent of the backmixing is a function of the mechanical energy imported into the system. To maintain a plug flow for achieving the high conversion requires the use of a lower gas input into the reactor, which in turn reduces the reactor productivity significantly.
It is believed that a significant improvement of the optimization of the SBCR for the Fischer-Tropsch synthesis is achievable using the concepts disclosed herein.
The flow pattern of the gas phase in the reactor can be described by the gas Peclet number, which has the form PeG=UGL/DG, where UG is the superficial gas velocity, L is the expanded slurry bed height, and DG is the dispersion coefficient. The dispersion coefficient is a function of the superficial gas velocity, gas holdup, and the reactor diameter. The gas Peclet number increases with the increase of gas velocity and the reactor aspect ratio, L/D. The change of the gas Peclet number with the superficial gas velocity at three reactor aspect ratios is shown in FIG. 1. As shown in FIG. 1, for a given reactor aspect ratio, the gas Peclet number decreases with the increase of the superficial gas velocity. If the reactor has a small aspect ratio, the gas phase will be in the well-mixed flow at most of the commercial gas flow rate. For a large aspect ratio reactor, the gas phase will be in the well-mixed regime at high gas velocity while in the plug flow regime at low gas velocity. The superficial gas velocity required to achieve the well-mixed gas flow decreases with the decreasing of reactor aspect ratio.